1. Field of the Invention
The present invention relates to a paper transport device for a thermal printer with a thermal head, especially to a color thermal printer wherein recording paper is transported alternately in a recording direction and in the opposite direction through the thermal head while the thermal head records one color frame at one transport in the recording direction, to record at least three color frames in a recording area.
2. Background Arts
In a color thermal printer, at least three color frames are sequentially recorded in a recording area on the recording paper. To prevent the color failure, accurate registration between the color frames is required, so the positioning of the recording area relative to a heating element array of the thermal head should also be accurate. Especially where the recording paper is transported repeatedly back and forth through the thermal head, a feed roller device consisting of a capstan roller and at least a press roller is disposed in proximity to the thermal head for more accurate control on the paper transportation.
There are two types in those thermal printers having the feed roller device: one has a stationary platen and a thermal head movable between a pressing position for pressing its heating elements on the recording paper and a retracted position, and the other has a stationary thermal head and a platen movable between a pressing position for pressing the recording paper onto the heating elements and a retracted position. The former type using the movable thermal head has a problem in that the load applied to the recording paper varies more largely by the contact and removal of the thermal head, compared with the load variation due to the contact and removal of the platen, so that the load variation is more likely to cause color failure or unexpected density variation. On the other hand, in the latter type using the movable platen, when the recording paper initially passes through between the thermal head and the platen, the leading end of the recording paper tends to strike against the thermal head and sometimes get jammed since the thermal head is disposed closer to the paper transport path.